1. Field of the Invention
The present invention relates to a receiver apparatus for collecting concentrated solar radiation.
2. Description of the Prior Art
In conventional thermal receiver systems concentrated solar energy is captured by an appropriate fluid and transformed into thermal or heat energy in a receiver apparatus. The fluid is subsequently collected and used to drive thermal devices, e.g., steam generators and the like. Fluids employed in conventional receiver apparatuses must have heat transfer and thermal storage properties to match those of solar energy systems.
Molten salts employed as fluids to capture thermal energy have been utilized effectively in solar energy systems. A salt, as used in this invention, is any of a class of compounds that are formed by the replacement of one or more hydrogen atoms of an acid with elements or groups and that usually ionize in a solution. Molten salts are known to be thermally stable, modestly-corrosive, to have high energy transfer densities, and possess thermal properties in the operating temperature range of conventional solar energy systems, i.e., between 100.degree.-1200.degree. C.
Molten salts are also non-reactive when exposed to air, thereby dispensing with the need to design, operate, and maintain air-tight, leak-proof systems. Additionally, salts have low vapor-pressures which minimize costs associated with high pressure containment systems. Finally, salts are inexpensive and readily available in abundant quantities.
There are numerous salts and salt mixtures which comply with the above requirements and are therefore used in solar energy applications. The salts which are known to have the greatest utility are nitrates, carbonates, chlorides, sulfates, and bromides of the alkali metals and alkaline earths. However, there are other salts known in the art which are useful in solar energy systems, such as those disclosed by Mar and Carling in their article entitled "The Application of Molten Salts to Solar Large Power Systems" published in 1980.
In typical closed system receivers, molten salt is pumped through a tube, the outside of which is irradiated by solar energy. As the tube becomes hotter than the salt, the latter cools the tube; thus, energy is drawn away from the tube. It has been demonstrated that if energy is imparted directly into the salt, receiver efficiency can be improved by as much as 10%. The principles of direct absorption prompt the salt to flow through the concentrated solar flux where the energy is absorbed directly into the blackened salt.
Molten salts used in the state-of-the-art direct absorption receivers are typically blackened by the addition of carbon black or another black particle agent. The application of agents to salts is desireable because it is well known by those skilled in the art that a darker surface captures or absorbs a greater amount of thermal energy than a lighter surface. The thermal energy is absorbed by the particle agent subsequently heating up the molten salt. Due to the overall particle surface area in contact with the salt, thermal energy is transferred to the salt. Moreover, this arrangement allows solar energy to be absorbed by particles deep within the salt.
In solar receivers, the proper amount of agent must be uniformly and continually mixed with the salt. Often it is difficult to achieve a uniform mixture, i.e., a mixture in which the agent is suspended in the salt solution because the solution, tends to separate from the agent due to buoyancy forces and gravity. When the agent is non-uniformly mixed throughout the fluid, the ability of the receiver to capture thermal radiation is seriously limited because the salt becomes too clear and solar radiation passes through without being absorbed. These mixture limitations additionally restrict the operation of direct absorption molten salt receivers, which causes substantial maintenance and other costs.
Another consideration affecting homogeneous agent and salt solution mixture is the flow orientation of the fluid. Typical state-of-the-art direct absorption receivers are designed so that the salt mixes in a relatively turbulent manner. To this end, the temperature at the outer surface of the flowing salt substantially equals the bulk temperature of the salt. When the outer surface temperature is comparable or equal to that of the bulk of the receiver's fluid, it is highly possible for an excessively high amount of thermal energy to escape to the outside environment by means of convection. In salt cooled tube receivers the temperature of the surface of the tube has been known to be 50.degree. C. hotter than the temperature of the bulk salt, which dramatically increases the possibility of receiver heat losses. Under these conditions, the ability to control losses is limited and the output efficiency of the receiver is diminished.